174 EUTHERIA 
divisions which are called “orders.” In the main zoologists 
are now of accord as to the general number and limits of these 
divisions among the existing forms, but the affinities and relation- 
ships of the orders to one another are far from being understood, and 
there are very many extinct forms already discovered which do not 
fit at all satisfactorily into any of the orders as commonly defined. 
Commencing with the most easily distinguished, we may first 
separate a group called Edentata, composed of several very distinct 
forms, the Sloths, Anteaters, and Armadillos, which under great 
modifications of characters of limbs and digestive organs, as well as 
habits of life, have just enough in common to make it probable that 
they are the very specialised survivors of an ancient group, most 
of the members of which are extinct, although the researches of 
paleontology have not yet revealed them to us. The characters of 
their cerebral, dental, and in many cases of their reproductive organs 
show an inferior grade of organisation to that of the generality of 
the subclass. The next order, about the limits of which there is no 
difficulty, is the Sirenia,—aquatic vegetable-eating animals, with 
complete absence of hind limbs, and low cerebral organisation,— 
represented in our present state of knowledge by but two existing 
genera, the Dugongs and Manatees, and by a few extinct forms, 
which, though approaching a more generalised mammalian type, 
show no special characters allying them to any of the other orders. 
Another equally well-marked and equally isolated, though far more 
numerously represented and diversified order, is that of the Cetacea, 
composed of the various forms of Whales, Dolphins, and Porpoises. 
In aquatic habits, external fish-like form, and absence of hind limbs, 
they resemble the last, though in all other characters they are 
as widely removed as are any two orders among the Eutheria. 
All the remaining orders are more nearly allied together, the 
steps by which they have become modified from one general 
type being in most cases not difficult to realise. Their dentition 
especially, however diversified in detail, always responds to the 
formula already alluded to, and, although the existing forms are 
broken up into groups in most cases easy of definition, the discoveries 
already made in paleontology have in great measure filled up the 
gaps between them. 
Very isolated among existing Eutheria are the two species of 
Elephant constituting the group called Proboscidea. These, however, 
are now known to be the survivors of a large series of similar animals, 
Mammoths, Mastodons, and Dinotheres, which as we pass backwards 
in time gradually assume a more ordinary or generalised type ; and 
the interval which was lately supposed to exist between even these 
and the rest of the class is partially bridged over by the discovery 
in American Eocene and early Miocene formations of the gigantic 
Dinocerata, evidently offshoots of the great group of hoofed animals, 
